Dry line controller

ABSTRACT

The position of a dry line on a Fourdrinier wire is maintained at substantially the same location after a grade change for a fibrous product formed on the wire as before the change. In concert with control of the bone dry basis weight of the product and wire speed during the grade change, the stream flow of a fiber water slurry fed to a headbox spouting to the wire is controlled. Stream flow control is in response to indications of the bone dry basis weight of the product before and after the grade change, as well as the wire speed before and after the grade change.

United States Patent 1 1 3,926,719 Spitz 5] Dec. 16, 1975 DRY LINE CONTROLLER Perry et al., Computer Control of a Fine Paper Ma- 75 Inventor: David A. Spitz, Columbus, Ohio ggx g P VOL 48 6 965) [73] Assignee: Industrial Nucleonics Corporation, Thompson paper Machine Under Digital Columbus puter Control", The Paper Industry, May, I962 pp. 221 Filed: Jan. 25, 1972 Savas, Computer Control in the Paper Mill", TAPPI, 20535 Apr. I964, pp. l8lA-l88A.

U.S. C. Pn'rnary E -uny 'ner s Leon Bashofe 2 162/263; 235/1513, DZlF/g/OZ Assistant Examiner-M. Steven Alvo Attorney Agent or Fryer [58] Field of Search 162/198, 263, 252, 253, M. Lowe; Hem-y C Peterson 162/256, 258, DIG. 6; 235/l5l.3

[56] References Cited [57] ABSTRACT UNITED STATES PATENTS The position of a dry line on a Fourdrin ier wire is 3 649 444 3H9 Fmch Jr 62/253 X maintained at substantially the same location after a 3'676295 7/972 Rice [62/253 x grade change for a fibrous product formed on the wire alosvisoz 8/1972 M51;'i'Lii'lTTiillIIIIl 162/253 x as before the change- Come" with Comm of the 3,703,436 2/1970 Rice l62/l98 bone y basis Weight of the Product and Wire Speed OTHER PUBLICATIONS during the grade change, the stream flow of a fiber water slurry fed to a headbox spouting to the wire is controlled. Stream flow control is in response to indi cations of the bone dry basis weight of the product before and after the grade change, as well as the wire speed before and after the grade change.

THlCK H STOCK '28 S URCE US. Patent D MOTOR SPEED CONTROL FBSG BSPG c (VFPG vos'Ta VDSB VFPG FBSG BSPG

COMPUTER.

BDBW

QWKG

'H-UCK STOCK Sou ace FLOW DRY LINE CONTROLLER FIELD OF THE INVENTION The present invention relates generally to Fourdri- 5 nier machines, and more particularly, to a Fourdrinier machine wherein dry line position is controlled in response to indications of bone dry basis weight of a fibrous product formed by the machine and the speed of a Fourdrinier wire.

BACKGROUND OF THE INVENTION In a Fourdrinier machine for making a fibrous sheet product, such as paper, from a fiber water slurry, the slurry spouts from a headbox to a moving Fourdrinier wire. The slurry, while on the wire, dries in response to drainage through the wire. Drainage is in response to suction provided beneath the wire by table rolls. There is a substantially definite location, referred to as a dry line, where there is no free water on top of a mat formed by the fiber water slurry on the Fourdrinier wire. In all Fourdrinier machines, it is imperative that the dry line occur before the end of the wire so that the mat will be converted into a sheet which can be handled by the remainder of the machine. In many ma- 35 chines. a dandy roller is positioned toward the end of the Fourdrinier wire. In such machines, it is desired to control the position of the dry line so that it is maintained a predetermined distance in front of the dandy roll, i.e., between the headbox and the dandy roll. If the dry line is located in front of the dandy roll by an excessive amount, the mat will be excessively dry when it contacts the dandy roll and fibers from the mat will be deposited on the roll, with resulting deleterious effects on the sheet product. If the dry line occurs at a location such that the mat is excessively wet when it engages the dandy roll, the dandy roll alters fiber formation of the matv Because of these factors, it is desirable to control the position of the dry line on the Fourdrinier wire. During normal operation of the machine, the dry line has a tendency to remain at a relatively fixed location. The position of the line has a tendency to vary, however, from one location to another when a grade change of the fibrous product formed by the wire is commanded. The dry line position may also vary significantly during non-grade change operation, if substantial changes in bone dry basis weight of the fibrous product and/or wire speed are commanded.

In the past, skilled machine operators have generally been employed to maintain the dry line position at substantially the same position before and after a grade change or other change materially affecting dry line. While this approach has been generally satisfactory, a great deal of experience is required on the part of the operator to provide the desired control and occasionally significant amounts of the sheet product are lost. One automated approach has been employed wherein the dry line is controlled as a function of the sheet bone dry basis weight, i.e., its fiber weight per unit area. It has been found. however, that dry line control as a function of only bone dry basis weight is frequently ineffective; in certain instances, it has been noted that the dry line moved many feet from the beginning to the end of a grade change.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention. l have discovered that the position of the dry line can be maintained substantially constant by controlling the flow rate of the fiber water slurry fed to the headbox, and hence the wire. in response to indications of the bone dry basis weight of the product formed by wire and the wire speed. Bone dry basis weight is important in controlling the dry line because mat thickness, which varies directly with bone dry basis weight. effects water drainage. Water drains from a thin mat more readily than from a thick mat, because water from the upper surface ofa thick mat has a greater solid volume to pass through than water on the upper surface of a thin mat, Water drainage is a direct function of wire speed because wire speed is related to the degree of vacuum placed on the bottom surface of the mat by table rolls located beneath the Fourdrinier wire. The greater the speed of the wire. the greater the vacuum exerted by the table rolls, to increase drainage. While wire speed also affects total drainage from the wire because of its effect on the time the mat is on the wire, this is a secondary effect that is much less significant than the primary wire speed effect on table roll vacuum.

During a grade change, a number of actuators for the paper making machine are controlled in steps. In one computer controlled system, the magnitude and duration of the several steps are ascertained as disclosed in the copending, commonly assigned application of James S. Rice, Ser. No. 8,383, filed Feb. 3, I970 and now abandoned. To prevent the dry line from changing relative to the beginning and end of a grade change (but not necessarily during the grade change) and in other instances where there is a significant change in a set point value of bone dry basis weight or wire velocity, each step of the stream fiow is computed in accordance with the present invention as:

BSPG

where:

DQWG stream flow change during each step.

QWSI-I stream flow set point prior to the change, i.e. for a grade change, the set point for the grade being formed before the grade change began,

TGKG the number of increments required to make the change,

FBSG bone dry basis weight set point prior to the change, i.e. for a grade change, bone dry basis weight set point for the grade before the grade change is started,

BSPG bone dry basis weight set point subsequent to the change, i.e. for a grade change, bone dry basis weight set point for the grade after the grade change has been completed,

VFPG wire speed set point after the change,

VDSB wire speed set point prior to the change,

COVG a constant determined from a theoretically derived curve fit of the vacuum profile between the table rolls and wire, as a function of wire velocity. and

COBG a constant determined from a theoretically derived curve fit of the vacuum profile between the table rolls and the wire. as a function of bone dry basis weight.

Calculations and data I have employed indicate that the values of C080 and COVG are approximately 0.4 and 0.8 over an extremely wide range of wire speeds, bone dry basis weights. and mat freeness. i.e.. the ability of the mat to transfer water through the wire. Equation l is valid only if a refiner feeding thick stock into the machine is operating in the same condition before the grade change as after. If the refiner operation is altered in conjunction with grade change, the alteration should be performed after the remainder of the grade change operations have been performed. In response to alterations of the refiner operation, the operator maintains the dry line position substantially constant by manually controlling stream flow to the headbox until the dry line returns to its initial location.

It is. accordingly. an object of the present invention to provide a new and improved system for and method of controlling dry line position.

Another object of the invention is to provide a new and improved system for and method of maintaining the position of a dry line at substantially the same location after a substantial change in a property of a fibrous sheet product has been effected as before the change.

Another object of the present invention is to provide a new and improved system and process for maintaining the position of a dry line at substantially the same location after a grade change has been performed as before the change.

Another object of the invention is to provide a system for and method of automatically maintaining a dry line position substantially constant by utilizing virtually identical analytic approaches with many different fibrous sheet producing machines.

The above and still further objects, features and ad vantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a schematic diagram of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to the FIGURE wherein a thick slurry of water and fiber is fed by source ll, which may be a paper making refiner, to headbox 12 via conduits l3 and 14, between which is connected pump 15. The fiber-water slurry in headbox l2 spouts from slice [6 thereof onto moving Fourdrinier wire 17. Beneath Fourdrinier wire 17 are table rolls l8 and flat boxes 18a which provide a vacuum or sunction effect on the lower surface of wire 17 to provide drainage for the mat of fiber and water formed on the upper surface of wire 17. Toward the end of wire 17 remote from slice 16 is dandy roll 19.

White water drained from wire 17 is caught in pit 21 and fed back, via pipe 22, pump 15, and conduit 14, to the inlet of headbox 12. The white water in conduit 22 dilutes the thick slurry fed to pump by conduit 13. Flow of fluid in pipe 14, and therefore steady state flow from slice 16 to wire 17, termed the stream flow, is controlled by valve 23 in line 14. The flow of thick stock in conduit 13, which determines the bone dry basis weight, i.e.. fiber weight per unit area. of the sheet formed by wire 17, is controlled by valve 24 in line 13.

Valves 23 and 24 are respectively driven by actuators 25 and 26 to set point values commensurate with desired stream flow and bone dry basis weight for a particular grade. To these ends, flow meters 27 and 28 are provided in lines 13 and 14. If the flow meters are not used, the positions of the valves may be used to alternatively provide flow indications. Flow meters 27 and 28 derive signals which are compared with set point sig nals for the flows in conduits 13 and 14 in subtracting nodes 29 and 30, respectively. Difference output signals derived from nodes 29 and 30 are fed to actuators 26 and 25, respectively, to provide negative feedback loops.

Wire 17 is driven by motor 32 at a forward speed determined by a set point signal fed to motor speed control circuit 33.

The set point input signals for node 29 and motor speed controller 33, are derived by general purpose digital computer 34 in a manner indicated in the previously referenced Rice application. The set point signal for node 30 is derived by computer 34 in accordance with the present invention. Computer 34 responds to grade indicating input signals to derive the various set points. During a grade change, the various set points are changed incrementally, i.e., in steps, by computer 34 as trajectories for controlling valves 23 and 24, as well as motor 32. The number of steps required for a particular grade change is determined by computer 34 as described in the referenced Rice application. From indications of the grade of the product being produced prior to the change, and the grade to be produced after the change has been performed, computer 34 responds to signals stored in the memory thereof to determine the values of: QWSH, TGKG, FBSG, BSPG, VFPG, and VDSB, where:

QWSH stream flow set point prior to the change, i.e., the set point for the grade being previously formed,

TGKG the number of increments required to make the grade change,

F BSG the bone dry basis weight set point for the grade being formed prior to the change. as stored in memory,

BSPG bone dry basis weight set point subsequent to the change, i.e.. bone dry basis weight for the grade of paper after the grade change has been completed,

VF PG wire speed set point after the grade change,

and

VDSB wire speed set point prior to the grade change.

From these values, as well as predetermined, stored values for C036 and COVG, constants determined from theoretical curve fits of the vacuum profile between the table rolls l8 and wire 17, as functions of bone dry basis weight and wire velocity, respectively, the change in stream flow, DQWG, during each step of the grade change is determined by computer 34 in accordance with Equation l supra. The computer 34 is programmed in a manner well known to those skilled in the art to solve the equation. The values of DQWG increment the previous value for the set point of valve 23 in the computer so that a trajectory of values QWKG for valve 23 is fed by computer 34 to node 30 during grade change. Thereby. the value of QWKG during the lth step of a grade change having TGKG steps is:

QwKG= hD0wo+QwsH (21. The computer successively feeds each of the incremented QWKG values of stream flow set point to node 30. Concomitantly with the feeding of incremented stream flow set point values to node 30, computer 34 feeds incremental values for the set points of thick stock flow and motor speed to node 29 and motor speed control 33.

From Equation l it is seen that if the speed of wire 17 is not changed from the beginning to the completion of a grade change and if there is no change in the bone dry basis weight of the sheet formed on wire 17 from the beginning to the end of the grade change, there is no change in the flow of slurry fed by conduit 14 to headbox 12. Relative changes in the speed of wire 17 from the beginning to the end of a grade change have a greater effect on changes of stream flow than relative changes in bone dry basis weight because of the higher value of the COVG coefficient than the COBG coefficient.

Values for the exponential coefficients COVG and COBG of approximately 0.8 and 0.4, respectively, have been found to provide reliable results in the great majority of machines with which the present invention has been employed. In certain instances, however, it has been found that there has been some movement of the dry line from its position before the grade change began to its position after the grade change has been completed. In such instances, it has been found that Equation I.) can be tuned by first observing its effect on the dry line position during speed changes, adjusting COVG, then observing its effect on the dry line position during grade change and adjusting COBG. If, during a speed increase, the dry line moves upstream, COVG should be increased. Once COVG has been set, dry line change with a basis weight change and small speed change should be observed. if the basis weight increases, and the dry line moves upstream, COBG should be decreased. The new values of COBG and COVG are stored in the computer memory.

While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. While the system and method have been described specifically with regard to grade change, it is to be understood that if there is a significant on grade change in the set points for wire speed and/or bone dry basis weight, stream flow set point trajectories can be, and are preferably, calculated by computer 34 in response to the new and 6 old set point values of bone dry basis weight and/or wire speed. I claim:

1. A system for maintaining the position of a dry line on a Fourdrinier wire at substantially the same location after a change in a fibrous product grade formed on the wire as before the change, said wire being responsive to the same type of thick stock throughout the change. said fibrous product being formed in response to a fiber water slurry fed to the wire and that is relatively dilute compared to the thick stock, comprising a first control for changing the fibrous product grade, said first control comprising means for deriving signals indicative of values for bone dry basis weight of the product before and after said change by said first control and indicative of values for wire speed before and after said change, said changes in bone dry basis weight and wire speed resulting in said change in the fibrous product grade, means responsive to said bone dry basis weight signals for incrementally controlling bone dry basis weight of the product, and means responsive to said wire speed signals for incrementally controlling the speed of the wire, a second control comprising means for incrementally controlling the flow rate of the fiber water slurry being fed to the wire in response to the signals indicative of the wire speed and bone dry basis weight before and after the change, all of said incremental control means being activated concomitantly and with such magnitude that the dry line position before the change is substantially the same as after the change, said second control for incrementally controlling the fiber flow rate includes means for computing:

OBG QW SH C FBSG [(W) where:

DQWG stream flow change during each step,

QWSH stream flow set point prior to the change,

TGKG the number of increments required to make the change,

FBSG bone dry basis weight set point prior to the change,

BSPG bone dry basis weight set point subsequent to the change,

VFPG wire speed set point after the change,

VDSB wire speed set point prior to the change.

COVG a constant determined from a curve fit of the vacuum profile between the table rolls and wire, as a function of wire velocity; and

COBG a constant determined from a curve fit of the vacuum profile between the table rolls and the wire, as a function of bone dry basis weight.

2. The apparatus of claim I wherein said computing means includes means for storing signals representing the fractional powers for the bone dry basis weight and speed ratios, said signals respectively representing the values substantially equal to 0.4 and 0.8. 

1. A SYSTEM FOR MAINTAINING THE POSITION OF A DRY LINE ON A FOURDRINIER WIRE AT SUBSTANTIALLY THE SAME LOCATION AFTER A CHANGE IN A FIBROUS PRODUCT GRADE FORMED ON THE WIRE AS BEFORE THE CHANGE, SAID WIRE BEING RESPONSIVE TO THE SAME TYPE OF THICK STOCK THROUGHOUT THE CHANGE, SAID FIBROUS PRODUCT BEING FORMED IN RESPONSE TO A FIBER WATER SLURRY FED TO THE WIRE AND THAT IS RELATIVELY DILUTE COMPARED TO THE THICK STOCK, COMPRISING A FIRST CONTROL FOR CHANGING THE FIBROUS PRODUCT GRADE, SAID FIRST CONTROL COMPRISING MEANS FOR DERIVING SIGNALS INDICATIVE VALUES FOR BONE DRY BASIS WEIGHT OF THE PRODUCT BEFORE AND AFTER SAID CHANGE BY SAID FIRST CONTROL AND INDICATIVE OF VALUES FOR WIRE SPEED BEFORE AND AFTER SAID CHANGE, SAID CHANGES IN BONE DRY BASIS WEIGHT AND WIRE SPEED RESULTING IN SAID CHANGE IN THE FIBROUS PRODUCT GRADE, MEANS RESPONSIVE TO SAID BONE DRY BASIS WEIGHT SIGNALS FOR INCREMENTALLY CONTROLLING BONE DRY BASIS WEIGHT OF THE PRODUCT, AND MEANS RESPONSIVE TO SAID WIRE SPEED SIGNALS FOR INCREMENTIALLY CONTROLLING THE SPEED OF THE WIRE, A SECOND CONTROL COMPRISING MEANS FOR INCREMENTALLY CONTROLLING THE FLOW RATE OF THE FIBER WATER SLURRY BEING FED TO LTHE WIRE IN RESPONSE TO THE SIGNALS INDICATIVE OF THE WIRE SPEED AND BONE DRY BASIS WEIGHT BEFORE AND AFTER THE CHANGE, ALL OF SAID INCREMENTAL CONTROL MEANS BEING ACTIVATED CONCOMITANTLY AND WITH SUCH MAGNITUDE THAT THE DRY LINE POSITION BEFORE THE CHANGE IS SUBSTANTIALLY THE SAME AS AFTER THE CHANGE, SAID SECOND CONTROL FOR INCREMENTALLY CONTROLLING THE FIBER FLOW RATE INCLUDES MEANS FOR COMPUTING:
 2. The apparatus of claim 1 wherein said computing means includes means for storing signals representing the fractional powers for the bone dry basis weight and speed ratios, said signals respectively representing the values substantially equal to 0.4 and 0.8. 